The radio antenna is the link between a transmitter/receiver and the outside world. The importance of an antenna having good radiation characteristics cannot be overemphasized. This is particularly true for CB applications in which the power of the CB transmitter is quite limited, by law, and in which substantially all transmitters are designed to operate at maximum legal power.
One good way, therefore, to increase the effectiveness of transmission is to utilize an antenna which radiates efficiently. The capability of an antenna and the efficiency with which it radiates is typically referred to as the gain of an antenna. In discussing gain, it is common to use a reference for comparison. One such reference is a half-wave dipole antenna. Thus, the gain of an antenna is often characterized by comparing its radiation pattern to that of the reference--a half-wave dipole antenna.
It is of course evident that the higher the gain of an antenna the more efficient and effective will be its transmission characteristics, assuming of course proper impedance matching with the transmission line.
Antennas may also be characterized by their directivity, being either omni-directional or directional. One technique utilized to increase the gain of an antenna is to focus its radiation pattern, i.e., make it directional. Such "beam" antennas do exhibit high gain in their direction of maximum radiation. This gain is obtained by "focusing" the radiation pattern in one direction, and by reducing sensitivity in directions other than the preferred direction. These directional antennas are typically utilized with antenna rotors in order to permit communications in different directions.
The size of such directional antennas, particularly for use in CB operations, is quite large and such antennas are also expensive. For this reason, and because directivity is not usually desired, omni-directional antennas are used in a large majority of the installations of base station antennas. The most common of these antennas include quarter-wave and five-eighths wave ground plane antennas, and half wave antennas.
It is generally recognized, that of the commonly used CB omni-directional vertical antennas, the five-eighths wave provides the most gain. For example, compared to a half-wave dipole, a quarter-wave ground plane antenna has a lower gain, -1.8 dB, while a five-eighths wave antenna has a higher gain, +1.2 dB. Extending the length of the antenna beyond five-eighths wave does not, as a practical matter, produce greater gain and, in fact a three-quarter wave antenna has less gain than the half-wave dipole reference.
One of the problems associated with quarter-wave and five-eighths wave antennas is the need for a ground plane. Particularly in CB operations, such a ground plane is typically provided by quarter-wave radially extending elements. In the CB range, these elements are approximately nine feet long and, therefor these antennas require sustantial lateral or horizontal space, more than 18 feet, as can be appreciated. These ground plane antennas are also strongly affected by the surrounding environment such as buildings, guy wires, and other interfering masses.
It would be desirable, therefore, to be able to provide an omni-directional antenna with improved gain characteristics while simultaneously minimizing the space requirements for such an antenna. Such an antenna would then provide good gain characteristics with minimum space requirements. Furthermore, if such an antenna could be built relatively simply and inexpensively, there would result a highly desirable and usable product.